Final answer:
Cahokia, near modern St. Louis, is the Mississippian site with North America's largest earthen mound, Monk's Mound. Its population might have surpassed 10,000 residents, peaking around 1100 CE before declining around 1300 CE.
Step-by-step explanation:
The Mississippian site near St. Louis known for having the largest earthen mound in North America and a significant population during its peak is Cahokia. This ancient city flourished around 1100 CE and may have supported a population exceeding 10,000 inhabitants, with some estimates suggesting up to 30,000 people. The centerpiece of Cahokia is the impressive Monk's Mound, a massive structure roughly ten stories high, covering 13.8 acres at its base. This mound served as the focal point of the city and possibly housed a grand temple or the residence of the ruling chief. Despite its prominence, Cahokia began to decline around 1300 CE, possibly due to the region's inability to sustain the large population.